As barrier materials with regard to oxygen, water vapor and the like, attraction has been focused on a gas barrier film obtained by forming, on a film base material, a metal and/or a metal oxide such as aluminum, aluminum oxide or silicon oxide, by a method such as vacuum deposition, sputtering, ion plating or chemical vapor deposition (CVD method). Such a gas barrier film is generally obtained by depositing an inorganic thin film on a surface of a base material composed of a biaxially-stretched polyester film excellent in transparency and rigidity. When such a film is used as it is, the deposited layer is susceptible to rubbing in use, and its use as a packaging film may involve the cracking of the inorganic thin film as a result of friction or elongation at the time of post-treatments including printing and laminating or at the time of incorporating contents, leading to the reduction of gas barrier properties. After hot water treatment such as boil treatment and retort treatment, partial delamination may be induced between the base material and the inorganic thin film, so that characteristics of the deposition film, such as gas barrier properties, may be significantly lowered.
Conventionally, in order to prevent the reduction of gas barrier properties, a method has been known in which between a polyester film serving as a base material of a deposition film and a deposited film, an under coat layer (primary layer) is provided which is, for example, any of various polyesters, polyurethane and acryls (For example, Patent Literature 1). Further, in order to prevent the reduction of gas barrier properties after hot water treatment, a coating layer containing any of a special acryl, polyurethane and polyester has been proposed (Patent Literature 2).
However, it is difficult for many of the deposition films described above to retain water resistance under high temperature (hot water resistance), in particular gas barrier properties after retort treatment. Retort-treating these films, for example, under retort treatment temperature condition of 135° C. would significantly lower their gas barrier properties. Thus, deposition films with gas barrier properties that would not be reduced even after treatment have been demanded.